Spring system having a variable spring characteristic

ABSTRACT

A spring system for providing an elastic force between two machine parts, including concentric compression and tension springs disposed in parallel within a housing, with one pair of corresponding ends of the springs being connected by a movable coupling piece, and the other pair of corresponding ends being connected to a stationary stop and a connecting element, whereby a movable stop permits variation of the effective point of connection between the compression spring and machine part connected therewith.

The invention relates to a spring system comprising at least two mainlyparallel, e.g. concentric, springs, wherein at least one spring, havinga variable spring characteristic, is connected at its one end with oneof two parts, between which an elastic force is to be introduced, and atleast one other spring is connected at its one end with the other part.

It is known that the spring characteristic of a spring, i.e. the travelof the spring as a function of the elastic force, can be changed byaltering the number of the effective turns of the spring, e.g. by meansof a clamp which fixes a number of the spring's turns in relation toeach other. The elastic force corresponding to a given travel of thespring can thereby be varied, e.g. in a mechanism in which a springacting as a power spring actuates two parts to move a certain distancetowards or away from each other at different loads, i.e. by exertingvarious spring forces, or in a spring suspension system in which variousloads are absorbed, the travels of the spring being unchanged.

If, for example, the same spring is in this way to be made to exertdifferent elastic forces within a predetermined interval, e.g. 50-100kp, with a constant, likewise predetermined, spring travel, a fairlylong and bulky spring must be used, which both permits the deformationcorresponding to the desired spring travel at the small force, e.g. 50kp, and by reduction of the number of effective turns limits thedeformation to the same value at the large force, e.g. 100 kp. At theelastic force interval of 50-100 kp chosen as example the slope of thespring characteristic must thus be changed by a factor of 2, and thefree spring length, i.e. the total length of the spring without loadwill be somewhat more than twice the desired spring travel. This meansthat a solution such as this is often not feasible owing toconsiderations of space.

From the U.S. Pat. No. 973,641 a spring system is known with twoconcentric springs, between which a tubular member is provided the endsof which are in mesh between the turns on the innermost and outermostspring, respectively. By turning the tubular member one can increase ordiminish the elastic force of the two springs at the same time, butunless other manipulations are made, this also increases or diminishesthe resulting elastic force of the system in the initial position, whichis often disadvantageous.

A purpose of the present invention is to produce a spring system whichin connection with a simple and compact structure and withoutreplacement of the spring permits a particularly large variation in thespring characteristic, i.e. a considerable increase or decrease of theelastic force within a predetermined interval at a predetermined springtravel and with unchanged elastic force in the initial position.

This is obtained by a spring system of the kind referred to aboveaccording to the invention by the other opposite ends of the springsbeing in firmly connection with each other and having a constantdistance from each other in the direction of the spring.

As a result of the mutual position and interconnection of the springs itis obtained that the resulting spring travel for the spring systemequals the sum of the travels of at least two springs, and as at leastone of these has a variable characteristic, a very considerable range ofvariation for the slope of the system's spring characteristic can beobtained depending on the choice of spring. The total obtainable springtravel that can be made to approach or even exceed the length of thespring system in the initial position also depends on the choice ofspring. Moreover, a constant elastic force is obtained in the initialposition, if the distance between the parts actuated by the springsystem is always the same in the initial position.

In principle, in a spring system according to the invention, anarbitrary number of springs can be used, but in many cases it issufficient to have two springs, which can be two tension springs, twocompression springs or one tension and one compression spring, and ofwhich one has a variable characteristic and preferably in the unloadedinitial position is weaker than the other spring, e.g. according to theinvention two concentric springs, at least one of which has a variablecharacteristic, said springs being connected with their respectivespring-actuated parts and with each other, the junction between one ofthe springs and either the appurtenant spring-actuated part or the otherspring being variable with a view to varying the number of effectiveturns of the spring in question.

A spring system according to the invention having, e.g. two springs withidentical characteristics, will in the initial position with anunregulated characteristic for the variable spring have a resultantsystem characteristic with a slope corresponding to twice the slope ofthe characteristic of the individual springs, because a force acting onone of the springs is transferred to the other spring via the connectionbetween the springs, and the springs are deformed to an equal extent.However, if the number of effective turns of the variable spring isdiminished, the slope of the characteristic of this spring and therebyalso the slope of the characteristic of the system decreases. If, forexample, the number of effective turns in the variable spring is reducedto zero, the slope of the spring characteristic of the system decreasesto half of the starting value. This possibility of variation is obtainedat a comparatively small length of the spring system in comparison tothe resultant spring travel, because this is equal to the sum of thetravels of the individual springs.

A greater variation in the slope of the characteristic of the system canhowever be obtained, if springs with different characteristics are usedin the system, that is, for example, two springs, one of which,preferably the spring with a variable number of effective turns, isweaker and has a steeper characteristic curve than the other spring. Thestrong spring can, if desired, have a certain initial tension.

When a spring system such as this is acted on by a gradually increasingload, the weak spring will be deformed most, and if the strong springhas an initial tension, to begin with only the weak spring is deformed,and its characteristic will then be practically the sole determiningfactor of the spring characteristic for the spring system as a whole. Inthat case the system characteristic will have a breaking-point at theload corresponding to the initial tension, at which load the strongspring begins to be deformed and thereby contribute to the total springtravel, which of course is again equal to the sum of the travels of theindividual springs, in this case the two individual springs.

Again a variable system characteristic with a comparatively large springtravel in relation to the length of the spring system has been obtained,and by the use of a strong spring with a preload the breaking-point inthe characteristic produced by this can be moved by alteration of thenumber of effective turns in the weak spring.

In both cases the load corresponding to a given total spring travel ortotal deformation of the individual springs depends on the proportion inwhich the deformation is divided between the springs, and thisproportion can be adjusted by variation of the number of effective turnsof the--at least in the initial position--weaker spring. If this numberis large, a given total travel corresponds to a comparatively smallload, and if the number of effective turns in the weak spring is low orperhaps zero, the given total travel corresponds to a greater load,because the strong spring then performs a greater part of or even thewhole of the total travel of the system.

This applies if, for example, the spring system is constituted by twotension springs or two compression springs and--particularly as regardsthe tension springs--is loaded only within the range in which Hooke'slaw applies. This can be secured, for instance, by restricting thetravel of a tension spring by means of a stop.

In a suitable embodiment of the invention the spring system isconstituted by tension springs and compression springs, e.g. twosprings, the weak spring being a compression spring and the strongspring being a tension spring.

In the case of a gradually increasing load of the spring system in thisembodiment it is also first and foremost the weak spring, i.e. thecompression spring, that is deformed, and the system characteristiccorresponds in essentials to the characteristic of this spring with aninitial slope corresponding to its slope, particularly if the strongertension spring has an initial tension. After a certain deformation ofthe compression spring, deformation of the stronger tension springbegins to appear, and the two springs jointly determine the systemcharacteristic, which also in this case may have a breaking-point andhas now a steeper course. When the load is further increased thecompression spring may at some point become completely compressed, andno longer contribute to the deformation or travel of the spring system.The system characteristic gets a new breaking-point and is then lesssteep, the slope being determined by the strong spring and thus beingless than the initial slope. The system characteristic and thereby theload corresponding to a given travel may also here be adjusted byvariation of the number of effective turns of the weak spring. If thisnumber is low, the strong spring will comparatively quickly come todetermine the course of the system characteristic, and a given travelwill correspond to a comparatively great load on the spring system.

In all cases a variation of the characteristic of the spring system ispermitted in connection with the spring system as a whole having acomparatively short length in relation to the spring travel, because thetravels of the separate springs comprised in the system and situated visa vis each other, e.g. concentric springs, are added together. The rangeof variation can be made greatest when the spring system is constitutedby a combination of tension and compression springs, because the lattercan be subjected to a more considerable load than tension springs and inthe present context do not lose their effect because they are completelycompressed. This means, for instance, that a compression spring with avariable number of effective turns can be combined with a considerablystronger tension spring having a very flat spring characteristic. Theresultant system characteristic will then take a very progressivecourse. Yet, it should be mentioned that in this context a compressionspring may be equated to a tension spring having adeformation-restricting stop, e.g. a drawing cable in the springconnecting the first and last turn of the spring and having a lengthcorresponding to the maximal spring length.

A spring system according to the invention can be realized with verysimple means, and can suitably be constructed with ordinary coilsprings.

By the terms "weaker" and "stronger" spring is here meant the springwhose characteristic at a given free spring length, i.e. length inunloaded condition, has respectively the greatest and the least slope.

The invention is explained below on the basis of a few examples ofembodiments with reference to the drawings, wherein:

FIG. 1 shows a practical embodiment of a spring system in a housing,

FIG. 2 is a sectional view along the line II--II in FIG. 1,

FIG. 3 depicts an adjusting clamp for the spring system shown in FIGS. 1and 2,

FIGS. 4a-4c depict various realizations of a spring system according tothe invention with different combinations of compression and tensionsprings, and

FIGS. 5a-5e depict an example of use of a system according to theinvention.

FIG. 2 shows two concentric coil springs in the form of a tension spring2 and a compression spring 3 in a joint housing 1. Of these two springsthe tension spring 2 has by way of example the greatest elastic force,and it may have an initial tension.

The spring 2 is accommodated in a tube 4, at the bottom of which it isattached by one of its ends, e.g. by means of a plug 5 screwed into thespring, through which plug extends a bolt 6, which also extends throughthe bottom of the tube and outside this bears, for instance, a drawingeyelet 7 that is screwed on. Inside the bolt 6 is fixed to the plug 5 bymeans of a couple of nuts 8.

At the opposite end of the spring 2 a plug 9 is also screwed in, andthis plug has a radial flange 10, via which the spring 2 is connected atthe said end with one end of the spring 3, which extends in thering-shaped space between the tube 4 and the housing 1 and at itsopposite end abuts against a ring 11 serving as a stop, the said ringbeing attached to the inner wall of the housing.

The two parts which are to be connected with each other via the springsystem are connected, respectively, with the housing 1, e.g. by means ofa couple of pins 12 attached externally to this housing, and with thedrawing eyelet 7, whereby the two parts, which are not shown but merelyindicated by A and B, are connected to one end of either one of thesprings 2 and 3, namely, the right end in FIG. 2, these springs beingconnected to each other at their other ends via the plug 9.

When the spring system shown in FIG. 2, which is a tension springsystem, is subjected to a load, e.g. by the drawing eyelet 7 beingsubjected to tension towards the right in FIG. 2, the spring 3, which asstated is in this case the weaker of the two springs, is deformed firstand mostly, and under compression of the spring 3 the tube 4 is moved acorresponding distance out of the housing 1. When the load has reached acertain size, e.g. when the turns of the spring 3 fit tightly againsteach other or shortly before depending on whether or not there is aninitial tension in the spring 2, the deformation of the stronger spring2 begins, and the extraction of the tube 4 from the housing 1 continues,the plug 9, which with its flange 10 abuts against and is held by thecompressed spring 3, keeping the end of the spring 2, into which it isscrewed.

During the first part of the extraction of the part 7 the springcharacteristic for the spring system as a whole has a coursecorresponding to the characteristic for the spring 3, as will directlybe seen, whereas the characteristic of the system during the last partof extraction movement, where the spring 3 can be wholly compressed, hasa course determined by the characteristic of the spring 2 and therebyless steep, if the spring travel is plotted along the axis of ordinatesand the spring force along the axis of the abscissa. The resultingspring travel for the spring system is equal to the sum of the travelsof the two separate springs, whereas the length of the housing 1 doesnot appreciably exceed the length of each of the two springs.

It can be seen from FIG. 1 that the spring system's housing 1 has slits14 along two opposite sides. These slits lie in pairs opposite eachother, a connecting line between two slits in a pair forming an anglecorresponding to the pitch of the turns of the spring 3 with thelongitudinal axis of the housing 1. In each such pair of slits aU-shaped clamp or fork, for example, can be inserted, as shown in FIG.3, where it is designated as 13. The internal distance between the legsof the clamp is slightly greater than the diameter of the tube 4, suchthat the clamp 13 can be inserted over this tube.

With this clamp 13 one can vary the spring characteristic for the spring3 and thereby for the spring system as a whole, as one can thereby varythe effective length of the spring 3. The insertion of the clamp 13corresponds to moving the stop at 11 towards the left. The closer to thelinking point 9, 10 between the two springs 2 and 3, i.e. the further tothe left in FIGS. 1 and 2 the clamp 13 is placed, the shorter becomesthe effective length of the spring 3, and the earlier the spring 2 comesto determine the characteristic of the system. Thus, the slope of thischaracteristic can be reduced by insertion of the clamp 13 further tothe left in FIGS. 1 and 2 and the slope can be increased by the clampbeing moved to the right, until the maximum steepness is obtained whenthe clamp 13 is removed completely.

As stated, the spring system in FIG. 2 is made up of a tension spring 2and a compression spring 3. If the flange 10 on the plug 9 is removed,the left end of the spring 3 in FIG. 2 is attached to the end wall ofthe housing 1, and the left end turn of the spring 2 is firmly connectedto the right end of the spring 3, and the stop ring 11 is removed, thespring 3 can also be a tension spring, and the effective part of thespring will lie to the right of the clamp 13, but the shown combinationof a tension and a compression spring is simpler both from the point ofview of construction and installation. An example of a system withtension springs alone is indicated schematically in FIG. 4b.

A spring system according to the invention will, for example, in manycases be able to be used as a prime mover for the raising and loweringmechanism in a piece of furniture of the kind having a seat that canswing up and down, in particular for handicapped persons. An example ofsuch mechanism, which is described in the Danish patent specificationNo. 146,223, is shown schematically in FIGS. 5a-5c. Other raising andlowering mechanisms for the stated purpose are, for example, describedin U.S. Pat. Nos. 3,479,086 and 4,185,335.

In the example of a seat lift shown in FIGS. 5a-5e, in which a springsystem according to the invention can be used as prime mover, the seatitself is designated as E.

From the one end position shown in FIG. 5a, where the seat E ishorizontal, the seat is moved via the position shown in FIG. 5b to theother end position shown in FIG. 5c, where the seat inclines downwardsso that the user can more easily leave it. The movement is obtained bymeans of a prime mover I, e.g. an electric motor with a worm drive,which when raising the seat E exerts tension on a spindle L, which via ahinge point H is connected to an arm G, which thereby turns a bearingarm A around the point 1. The bearing arm A is both connected to therear edge of the seat E and in a point D,D1, D2 connected to anotherbearing arm B, which at one end is connected to the front edge of theseat E and at its opposite end at K is connected to the housing of theprime mover I. Under the influence of the prime mover I the points K andH are thus made to approach each other when raising the seat E, whereasthey move away from one another when lowering the seat.

If instead of, e.g. an electric motor as the prime mover I a springsystem is used according to the invention, e.g. in the form shown inFIG. 2, this spring system in the initial position of the seat E asshown in FIG. 5a, in which the power requirement is large, will exercisea very great tension between the points K and H, when the user releasesa lock which arrests the seat in the lowered horizontal position, and asomewhat weaker tension when the seat approaches the other end positionshown in FIG. 5c. The tension exerted by the spring system between thepoints K and H can be adjusted and adapted according to the weight ofvarious users by means of the U-shaped clamp.

With the spring system as prime mover the chair is not dependent on theexistence of a source of energy, e.g. a battery or an electric socket.

However, there are also other uses for a spring system according to theinvention, and the use in a seat lift described above is only anexample. Thus, in many cases it will also be possible to use a springsystem according to the invention instead of a variable counterweight,e.g. in connection with cranes.

Also in apparatus or constructions in which a constant elastic force isdesired between two parts at a variable movement or mutual distance aspring system elaborated according to the present invention will be ableto be used.

FIGS. 4a-4c shows schematically three examples of spring systemsaccording to the invention. All three examples involve a system withthree parallel springs, placed vis a vis each other, and in each ofthese systems either the middle spring or the two exterior springs mayhave a variable spring characteristic. Moreover, it holds good for eachof the three systems that the two exterior springs can be replaced by asingle spring, which then preferably surrounds and is concentric withthe middle spring.

FIG. 4a shows a system with two exterior compression springs and atension spring between them. This system is both from the point of viewof construction and function analogous to the embodiment shown in FIG.2.

FIG. 4b shows a system with three tension springs analogous to the onesuggested in connection with the explanation of FIG. 2.

FIGS. 4a and 4b show tension spring systems, and FIG. 4c shows acompression spring system involving three compression springs.

I claim:
 1. A spring system for imparting an elastic force between twoparts comprising:(a) a housing; (b) at least first and second springsdisposed within the housing; (c) means carried by the housing forconnecting the first spring to one part; (d) a coupling piece movablewithin the housing and connecting one pair of corresponding ends of thefirst and second springs; (e) a stationary stop and a connecting elementdisposed within the housing, the stationary stop connecting the otherend of the first spring and the connecting element connecting the otherend of the second spring; (f) the connecting element being provided withmeans for connecting the element to the other part; and (g) a movablestop for varying the effective point of connection between the firstspring and part connected therewith.
 2. The spring system of claim 1wherein the housing includes two opposed longitudinal wall sections anda plurality of openings provided in each wall section, wherein eachopening in one wall section corresponds to an opening in the opposedwall section, and the movable stop being selectively insertable in anycorresponding pair of openings for engaging the first spring.